1. Field of the Invention
The present invention relates to photoelectric conversion devices and manufacturing methods thereof, the devices used for image input apparatuses such as digital cameras, video cameras, and image readers.
2. Description of the Related Art
In recent years, photoelectric conversion devices have been incorporated in image input apparatuses such as digital cameras, video cameras, and image readers. These photoelectric conversion devices include, for example, CCD image sensors and non-CCD image sensors, such as bipolar transistor image sensors, field effect transistor image sensor, and CMOS image sensors. In such photoelectric conversion devices, optical image information is converted into electrical signals, which are processed using various types of signal processing and displayed in a display device or recorded on a recording medium.
In order to obtain a high performance photoelectric conversion device, the area (pixel area) of a light-receiving surface of a photoelectric conversion element, which area is a light-receiving portion actually performing photoelectric conversion, should be decreased so that the number of photoelectric conversion elements is increased, and in addition, so that the chip size of the photoelectric conversion device is decreased.
As progress toward higher pixel density and reduction in chip size advances, the amount of light received by each photoelectric conversion element forming a pixel decreases as the area of the light-receiving surface is decreased, and as a result, the sensitivity of the device is degraded. In order to suppress this degradation in sensitivity, there is a well known technique in which microlenses are formed on a planarized surface of a protective film provided on the light-receiving surface, so that light is concentrated thereon.
For example, in Japanese Patent Laid-Open No. 10-107238, a manufacturing method of a solid-state image sensing device has been disclosed in which an on-chip lens is formed using an etch-back technique. In this manufacturing method, as shown in FIGS. 10A and 10B, first, a planarizing film 104 is formed on sensor portions 101 and a pad portion 102, and above the sensor portions 101 and the pad portion 102, color filters 103 are formed with the planarizing film 104 positioned therebetween. Subsequently, after a lens material 105 is applied, a lens pattern 106 is formed by patterning through a photolithographic and a thermal treatment step. Next, the entire surface is etch-backed by an etch-back amount 107, thereby forming an on-chip lens 108, as shown in FIG. 10B.
Using this manufacturing method, the formation of the on-chip lens 108 and the formation of an opening above the pad portion 102 can be simultaneously performed. In addition, when the difference between the amount removed by etching for the on-chip lens 108 and the amount removed for the opening above the pad portion 102 is decreased, damage done to the pad portion 102 can be reduced.
Concomitant with the trend toward higher pixel density and reduction in chip size, it has been increasingly required to provide interlayer lenses formed of a film having a refractive index different from that of an adjacent layer. For example, in Japanese Patent Laid-Open No. 2001-94086, a photoelectric conversion device has been disclosed in which light condensation efficiency can be improved even when the light-receiving surface is more finely formed and/or a great number of various films, such as a light shielding pattern and a wire pattern, are formed on the light-receiving surface.
As shown in FIG. 11, this photoelectric conversion device has a first wire pattern 203 having wires positioned above the element isolation region 202 located between adjacent photoelectric conversion elements 201, a first insulating film 204 covering the first wire pattern 203, a second wire pattern 205 provided on the first insulating film 204 and having wires positioned above the element isolation region 202, a second insulating film 206 covering the second wire pattern 205, and microlenses 207 provided on the second insulating film 206. The insulating layers 204 and 206 are applied in two steps. First, a layer of predetermined thickness is applied over the wire pattern (203 and 205, respectively) to form concave portions in the areas between the wires (i.e., the areas over the photoelectric conversion elements 201). Then, an additional layer is applied and made planar on its upper surface to form first and second interlayer lenses 208 and 209 in the optical paths between the microlenses 207 and the light-receiving surfaces of the corresponding photoelectric conversion elements 201. Thus, the step structures provided by the wire patterns 203 and 205 determine, at least in part, the shape of the first and second interlayer lenses 208 and 209.
According to Japanese Patent Laid-Open No. 11-040787, in a photoelectric conversion device which has a charge transfer portion for transferring photoelectric-converted charges and a transfer electrode provided above the charge transfer portion with an insulating film provided therebetween, a structure has been disclosed in which upward convex-shaped interlayer lenses are formed on a planarizing film.
However, according to the manufacturing method depicted in FIG. 11, a curved surface formed in the insulating film that forms the interlayer lenses is limited to having “peaks” above constituent elements of the pattern (e.g., 203) and “valleys” therebetween. Thus, the shape of the interlayer lens depends on the shape of the pattern and is also limited thereby. Accordingly, depending on the shape of the patterns, an interlayer lens having a desired light condensation efficiency may not be formed in some cases.
In addition, when interlayer lenses formed of a plurality of layers are combined with each other in order to improve the light condensation efficiency, the probability of light reflection occurring at the interface formed between layers having different refractive indexes increases as the number of layers forming the interlayer lenses is increased. Also, when the number of interfaces causing light reflection is increased, the number of light reflections increases accordingly. Hence, the amount of light incident on the light-receiving surface of the photoelectric conversion element is decreased, and as a result, the sensitivity of the photoelectric conversion device may be substantially decreased. In addition, in a structure having a monolayer wire, for example, as disclosed in Japanese Patent Laid-Open No. 11-040787, it is relatively easy to make the optical path length from the lens to the light-receiving portion small; however, in a photoelectric conversion device having a plurality of wire layers, the optical path length to the light-receiving portion tends to be increased, and hence the technical problem described above must be overcome.